Since the multitexturing extension GL_ARB_multitexture does not support texture coordinate replication for the additional texture units (in contrast to GL_SGIS_multitexture) what about an additional TexGen mode (as an extension GL_{EXT|ARB}_texgen_replicate), allowing the following (assuming GL_TEXTUREn_ARB, n>0 is active):
glTexGeni(GL_S, GL_REPLICATE,
GL_TEXTUREm_ARB);
or
glTexGeni(GL_ALL, GL_REPLICATE,
GL_TEXTUREm_ARB);
with m<n
or m!=n and m<=maxTextureUnits.